In accordance with increase of interest in environmental issues and development of info-communication equipment, sensors for various gases have been developed in recent years. By grafting the sensors with semiconductor technology, it is easy to manufacture the sensors, and the manufactured sensors have improved performance. A primary goal of all sensors is to increase sensitivity for improving performance, and an effort to achieve this goal has also been increased.
Meanwhile, since a semiconductor-type gas sensor in the related art includes a semiconductor thin film as a sensing material, there is limitation in sensitivity. For example, it is almost impossible to sense stable chemical materials such as carbon dioxide (CO2).
Accordingly, in the sensor for sensing harmful gases such as carbon monoxide (CO), carbon dioxide, and the like, an electrochemical method using a conductive method of a solution, an optical method by infrared absorption, and a method for measuring electrical resistance of nanoparticles or nanowire have been applied.
The electrochemical method is to measure current flowing in external circuits by electrochemically oxidizing or reducing target gas, or to use electromotive force generated from ions in gas phase dissolved or ionized in an electrolyte solution or a solid, acting on an ionic electrode, which has disadvantages in that a reaction rate is extremely low, gas sensing range and environment for using the sensor are limited, and the cost is also high.
In addition, the optical method by infrared absorption has an advantage to be rarely affected by other mixed gases or humidity; however, it has disadvantages such as a complicated device, a large size, and high cost.
In general, a chemical sensor has a structure for sensing gas by contact combustion, such that when the gas reacts with the sensor including a platinum wire as a catalyst, the sensor is capable of sensing the gas by using change in resistance of the platinum wire by endothermic reaction or exothermic reaction, to thereby have improved stability and sensitivity.
Meanwhile, as the relationship between contact reaction by chemical adsorption of a gas and electron density has been identified and an oxide semiconductor-type gas sensor has been recently developed and commercialized, the semiconductor-type gas sensor has been developed to be capable of sensing most gases including a combustible gas, which achieves miniaturization, cost reduction and improvement in reliability as compared to gas sensors according to other schemes.
As compared to other sensors that are required to be heated up to about 300° C. to detect nitrogen oxide, and the like, a gas sensor using carbon nanotube as one of the semiconductor-type gas sensor has an advantage in that sensitivity thereof is thousands of times higher than those of other sensors since the carbon nanotube is possible to be operated even at room temperature, and has nano-sized particles.
A gas sensor of measuring change in electrical resistance of a nanoparticle itself or a material coating the nanoparticle according to concentration of a gas to be measured, has been developed. When the nanoparticles are used, an area ratio to volume is significantly high, such that an effect into change in resistance with respect to total volume of an effect of a surface reaction according to change in gas concentration is significantly large, thereby making it possible to manufacture a sensor having significantly high sensitivity.
As described in Korean Patent No. 10-0655640 (Dec. 4, 2006), in the sensor using the nanoparticles or the nanowires according to the related art, electrical resistance is measured by connecting electrodes capable of measuring change in electrical resistance of nanomaterials only at specific portions by non-uniformly dispersing the nanomaterials on the surface, or flowing the nanomaterials on pre-patterned electrodes, or by using electrophoresis to be in contact with the electrodes.
The above-mentioned semiconductor-type gas sensor according to the related art has disadvantages in that physical and electrical connection between the nanomaterials and the electrodes is unstable, and the nanomaterials being in contact with the surface are affected by the surface in a gas sensing process.
Afterward, a suspended nanowire-based sensor is manufactured by adhering a nanowire onto electrodes spaced apart from the surface at a predetermined interval, that is, having a post shape by electrophoresis, or selectively growing the nanowires from one electrode to the opposite electrode. The existing suspended nanowire-based sensor has good sensitivity, but has poor contact between the nanowire and the electrodes, difficulty in controlling a manufacturing process, and requires high cost in manufacturing the sensor and long time for manufacturing the sensor. Therefore, the sensor has limitation in commercialization for mass-production of the sensor.